Display device and electronic device having the display device, and method for manufacturing thereof

ABSTRACT

To provide a display device including a thin film transistor in which high electric characteristics and reduction in off-current can be achieved. The display device having a thin film transistor includes a substrate, a gate electrode provided over the substrate, a gate insulating film provided over the gate electrode, a microcrystalline semiconductor film provided over the gate electrode with the gate insulating film interposed therebetween, a channel protection layer which is provided over and in contact with the microcrystalline semiconductor film, an amorphous semiconductor film provided over the gate insulating film and on a side surface of the microcrystalline semiconductor film and the channel protection layer, an impurity semiconductor layer provided over the amorphous semiconductor film, and a source electrode and a drain electrode provided over and in contact with the impurity semiconductor layer. The thickness of the amorphous semiconductor film is larger than that of the microcrystalline semiconductor film.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a display device and an electronicdevice using the display device. In particular, the present inventionrelates to a display device using a thin film transistor in a pixelportion and an electronic device using the display device.

2. Description of the Related Art

In recent years, techniques that are used to form thin film transistorsusing a semiconductor thin film (with a thickness of approximatelyseveral nanometers to several hundreds of nanometers) which is formedover a substrate with an insulating surface have been put into practicein many electronic devices. In particular, thin film transistors havebeen put into practical use as switching elements in a pixel portion ofa display device, and research and development has been activelyconducted.

As a switching element of a liquid crystal display device, a thin filmtransistor using an amorphous semiconductor film in a large panel or athin film transistor using a polycrystalline semiconductor film in asmall panel is used. As a method for forming a polycrystallinesemiconductor film, there is known a technique in which a pulsed excimerlaser beam is shaped into a linear laser beam by an optical system, andan amorphous semiconductor film is crystallized by being irradiatedwhile being scanned with the linear laser beam.

As a switching element of an image display device, a thin filmtransistor using a microcrystalline semiconductor film is used(Reference 1: Japanese Published Patent Application No. H4-242724,Reference 2: Japanese Published Patent Application No. 2005-49832, andReference 3: U.S. Pat. No. 5,591,987). In addition, as a method formanufacturing a thin film transistor for improvement of characteristicsof an amorphous semiconductor film, a method is known in which anamorphous silicon film is formed over a gate insulating film, and then ametal film is formed thereover, and the metal film is irradiated with adiode laser beam to modify the amorphous silicon film into amicrocrystalline silicon film (Non-Patent Document 1: Toshiaki Arai etal., SID 07 digest, 2007, pp. 1370-1373). According to this method, themetal film formed over the amorphous silicon film is provided to convertlight energy of the diode laser beam into thermal energy and should beremoved in a later step to complete a thin film transistor. That is, themethod is that in which the amorphous silicon film is heated only byconduction heating from the metal film, thereby forming amicrocrystalline silicon film that is a microcrystalline semiconductorfilm.

SUMMARY OF THE INVENTION

A thin film transistor using a polycrystalline semiconductor film hasadvantages in that mobility is two or more orders of magnitude greaterthan that of a thin film transistor using an amorphous semiconductorfilm, and a pixel portion of a display device and peripheral drivercircuits thereof can be formed over the same substrate. However, theprocess for crystallization of a semiconductor film is more complex thanthat in the case of using an amorphous semiconductor film. Accordingly,there are problems in that yield is decreased and cost is increased.

There is also a problem in that the surface of a microcrystallinesemiconductor film is likely to be oxidized. Therefore, when crystalgrains in a channel formation region are oxidized, oxide films areformed on the surfaces of the crystal grains and the oxide films becomeobstacles to carrier transfer, which causes a problem in that electriccharacteristics of a thin film transistor are impaired. In addition,there is a problem in that it is difficult to increase the thickness ofthe microcrystalline semiconductor film as compared to that of anamorphous semiconductor film or of a polycrystalline semiconductor filmand parasitic capacitance between a gate electrode and a sourceelectrode and/or a drain electrode is caused to be increased.

In terms of ease of production, a thin film transistor having aninverted staggered structure is promising as a switching element that isprovided in a pixel portion of a display device. From the viewpoint ofimprovement in aperture ratio of a pixel, while a thin film transistorhaving an inverted staggered structure is expected to have highperformance and to be downsized, there is a problem in that leakagecurrent (also referred to as off-current) flowing between a sourceregion and a drain region is increased when the thin film transistor isin an off state. Therefore, there is a problem in that it is difficultto downsize the size of the thin film transistor, to reduce a storagecapacitor, and to decrease power consumption.

In view of the foregoing problems, an object of the present invention isto provide a display device including a thin film transistor in whichthe decrease in yield and the increase in parasitic capacitance and inproduction cost are suppressed, high electric characteristics andreduction in off-current are achieved.

One feature of the present invention is a display device having a thinfilm transistor, including: a gate electrode provided over a substrate;a gate insulating film provided over the gate electrode; amicrocrystalline semiconductor film provided over the gate electrodewith the gate insulating film interposed therebetween; a channelprotection layer which is provided over the microcrystallinesemiconductor film and which is in contact with the microcrystallinesemiconductor film; an amorphous semiconductor film provided over thegate insulating film and on a side surface of the microcrystallinesemiconductor film and the channel protection layer; an impuritysemiconductor layer provided over the amorphous semiconductor film; anda source electrode and a drain electrode each provided in contact withthe impurity semiconductor layer. The thickness of the amorphoussemiconductor film is larger than the thickness of the microcrystallinesemiconductor film.

Another feature of the present invention is a display device having athin film transistor, including: a gate electrode provided over asubstrate; a gate insulating film provided over the gate electrode; amicrocrystalline semiconductor film provided over the gate electrodewith the gate insulating film interposed therebetween; a channelprotection layer which is provided over the microcrystallinesemiconductor film and which is in contact with the microcrystallinesemiconductor film; an amorphous semiconductor film provided over thegate insulating film and on a side surface of the microcrystallinesemiconductor film and the channel protection layer; an impuritysemiconductor layer provided over the amorphous semiconductor film; anda source electrode and a drain electrode each provided in contact withthe impurity semiconductor layer. The thickness of the amorphoussemiconductor film is larger than the thickness of the microcrystallinesemiconductor film, a part of the impurity semiconductor layer and apart of the amorphous semiconductor film are exposed outside the sourceelectrode and the drain electrode, and one of end portions of theimpurity semiconductor layer and one of end portions of the amorphoussemiconductor film are aligned with each other over the gate electrode.

Another feature of the present invention is a display device having athin film transistor, including: a gate electrode provided over asubstrate; a gate insulating film provided over the gate electrode; amicrocrystalline semiconductor film provided over the gate electrodewith the gate insulating film interposed therebetween; a channelprotection layer which is provided over the microcrystallinesemiconductor film and which is in contact with the microcrystallinesemiconductor film; an amorphous semiconductor film provided over thegate insulating film and on a side surface of the microcrystallinesemiconductor film and the channel protection layer; an impuritysemiconductor layer provided over the amorphous semiconductor film; asource electrode and a drain electrode each provided in contact with theimpurity semiconductor layer; an insulating film which is in contactwith the source electrode, the drain electrode, the impuritysemiconductor layer, and the amorphous semiconductor film; and a pixelelectrode which is formed over the insulating film and connected to oneof the source electrode and the drain electrode in a contact hole formedin the insulating film. The thickness of the amorphous semiconductorfilm is larger than the thickness of the microcrystalline semiconductorfilm.

Another feature of the present invention is a display device having athin film transistor, including: a gate electrode provided over asubstrate; a gate insulating film provided over the gate electrode; amicrocrystalline semiconductor film provided over the gate electrodewith the gate insulating film interposed therebetween; a channelprotection layer which is provided over the microcrystallinesemiconductor film and which is in contact with the microcrystallinesemiconductor film; an amorphous semiconductor film provided over thegate insulating film and on a side surface of the microcrystallinesemiconductor film and the channel protection layer; an impuritysemiconductor layer provided over the amorphous semiconductor film; asource electrode and a drain electrode each provided in contact with theimpurity semiconductor layer; an insulating film which is in contactwith the source electrode, the drain electrode, the impuritysemiconductor layer, and the amorphous semiconductor film; and a pixelelectrode which is formed over the insulating film and connected to oneof the source electrode and the drain electrode in a contact hole formedin the insulating film. The thickness of the amorphous semiconductorfilm is larger than the thickness of the microcrystalline semiconductorfilm, a part of the impurity semiconductor layer and a part of theamorphous semiconductor film are exposed outside the source electrodeand the drain electrode, and one of end portions of the impuritysemiconductor layer and one of end portions of the amorphoussemiconductor film are aligned with each other over the gate electrode.

Note that, in the display device of the present invention, the channelprotection layer may be one of a silicon nitride film and a siliconnitride oxide film.

Because of misalignment of the edge portions of the source electrode andthe drain electrode with the edge portions of the impurity semiconductorlayer and formation of the edge portions of the impurity semiconductorlayer outside the edge portions of the source electrode and the drainelectrode, the edge portions of the source electrode and the drainelectrode are apart from each other; accordingly, leakage current and ashort circuit between the source electrode and the drain electrode canbe prevented. In addition, an electric field can be prevented from beingconcentrated on the edge portions of the source electrode and the drainelectrode and the impurity semiconductor layer, and leakage currentbetween the gate electrode and the source electrode and/or the drainelectrode can be prevented.

In addition, an amorphous semiconductor layer is provided on a sidesurface of a microcrystalline semiconductor film and a channelprotection layer. Since the amorphous semiconductor layer is provided,the distance of the impurity semiconductor layer that serves as a sourceregion and a drain region can be long, and leakage current flowing theimpurity semiconductor layer can be reduced. Further, since theamorphous semiconductor layer is provided, the thickness between a gateelectrode and a source electrode and/or a drain electrode can be madelarge; therefore, parasitic capacitance generated between the gateelectrode and the source electrode and/or the drain electrode can bereduced.

Over the microcrystalline semiconductor film, a channel protection layeris provided in contact with the microcrystalline semiconductor film. Themicrocrystalline semiconductor film functions as a channel formationregion. The channel protection layer prevents oxidation of themicrocrystalline semiconductor film and functions as an etching stopperin manufacturing process of a thin film transistor. Since the channelprotection layer is provided in contact with the microcrystallinesemiconductor film, the thickness of the microcrystalline semiconductorfilm can be made small and the oxidation of crystal grains contained inthe microcrystalline semiconductor film can be prevented; therefore, athin film transistor which has high mobility, small leakage current, andhigh withstand voltage can be obtained.

Unlike a polycrystalline semiconductor film, a microcrystallinesemiconductor film can be directly formed on a substrate as amicrocrystalline semiconductor film. Specifically, a microcrystallinesemiconductor film can be formed using silicon hydride as a source gasand using a plasma CVD apparatus. The microcrystalline semiconductorfilm formed by the above method includes a microcrystallinesemiconductor film which contains crystal grains of 0.5 nm to 20 nm inan amorphous semiconductor. Thus, unlike in the case of using apolycrystalline semiconductor film, there is no need to provide acrystallization process after formation of a semiconductor film. Thenumber of steps in manufacture of a thin film transistor can be reduced;yield of a display device can be increased; and cost can be lowered.Plasma using a microwave with a frequency of 1 GHz or more has highelectron density, which facilitates dissociation of silicon hydride thatis a source gas. Therefore, compared to a microwave plasma CVD methodwith a frequency of several tens to several hundreds of megahertz, themicrocrystalline semiconductor film can be formed more easily and filmformation rate can be increased. Thus, the mass productivity of displaydevices can be increased.

In addition, thin film transistors (TFTs) are formed using amicrocrystalline semiconductor film, and a display device ismanufactured using the thin film transistors in a pixel portion and alsoin driver circuits. Because thin film transistors using amicrocrystalline semiconductor film each have a mobility of 1 cm²/V·secto 20 cm²/V·sec, which is 2 to 20 times greater than that of a thin filmtransistor using an amorphous semiconductor film, some of or all of thedriver circuits can be formed over the same substrate as the pixelportion to form a system-on-panel display.

The display device includes a liquid crystal element or a light-emittingelement (generally called a display element). In addition, the displaydevice includes a panel in which a display element is sealed, and amodule in which an IC and the like including a controller are mounted onthe panel. The present invention further relates to one mode of anelement substrate before the display element is completed in amanufacturing process of the display device, and the element substrateis provided with a means to supply voltage to the display element ineach of a plurality of pixels. An element substrate may be specificallyin a state where only a pixel electrode of a display element is formedor in a state after a conductive film to serve as a pixel electrode isformed and before the conductive film is etched into a pixel electrode,and any mode is possible.

Note that display devices in this specification refer to image displaydevices and light sources (including lighting devices). In addition,display devices include all of the following modules: modules providedwith a connector, for example, a flexible printed circuit (FPC), a tapeautomated bonding (TAB) tape, or a tape carrier package (TCP); modulesprovided with a printed wiring board at the end of a TAB tape or a TCP;and modules where an integrated circuit (IC) is directly mounted on adisplay element by a chip-on-glass (COG) method.

The present invention can provide a display device including a thin filmtransistor in which reduction in yield can be suppressed, an increase inparasitic capacitance and production cost can be suppressed, highelectric characteristics can be achieved, and off-current can bereduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C are cross-sectional views illustrating a method formanufacturing a display device of the present invention.

FIGS. 2A to 2C are cross-sectional views illustrating a method formanufacturing a display device of the present invention.

FIGS. 3A to 3C are cross-sectional views illustrating a method formanufacturing a display device of the present invention.

FIG. 4 is a cross-sectional view illustrating a method for manufacturinga display device of the present invention.

FIGS. 5A to 5C are top views illustrating a display device of thepresent invention.

FIG. 6 is a top view illustrating a microwave plasma CVD apparatus.

FIGS. 7A and 7B are diagrams illustrating a display device of thepresent invention.

FIGS. 8A and 8B are diagrams illustrating a display device of thepresent invention.

FIGS. 9A to 9C are diagrams illustrating a display device of the presentinvention.

FIG. 10 is a diagram illustrating a display device of the presentinvention.

FIGS. 11A to 11C are diagrams illustrating electronic devices includinga display device of the present invention.

FIG. 12 is a diagram illustrating an electronic device including adisplay device of the present invention.

FIG. 13 is a cross-sectional view illustrating a thin film transistorincluded in a display device of the present invention.

FIG. 14 is a cross-sectional view illustrating a thin film transistorincluded in a display device of the present invention.

FIG. 15 is a diagram illustrating a cross-sectional structure of atransistor for which simulation calculation is performed.

FIG. 16 is a diagram illustrating a current-voltage characteristic of atransistor structure shown in Embodiment Mode 8.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiment modes of the present invention will be describedwith reference to the accompanying drawings. However, the presentinvention can be implemented in various modes. As can be easilyunderstood by those skilled in the art, the modes and details of thepresent invention can be changed in various ways without departing fromthe spirit and scope of the present invention. Thus, the presentinvention should not be taken as being limited to the followingdescription of the embodiment modes.

Embodiment Mode 1

In this embodiment mode, manufacturing processes of thin filmtransistors used for a display device will be described with referenceto FIGS. 1A to 1C, FIGS. 2A to 2C, FIGS. 3A to 3C, FIG. 4, and FIGS. 5Ato 5C. FIGS. 1A to 1C, FIGS. 2A to 2C, FIGS. 3A to 3C, and FIG. 4 arecross-sectional views illustrating manufacturing processes of thin filmtransistors, and FIGS. 5A to 5C are top views each illustrating aconnection region of a thin film transistor in one pixel of a displaydevice and a pixel electrode.

A thin film transistor having a microcrystalline semiconductor film,which is of an n type, is more suitable for use in a driver circuit thanthat of a p type because it has higher mobility. It is desired that allthin film transistors formed over the same substrate have the samepolarity, in order to reduce the number of steps. Here, description ismade using n-channel thin film transistors.

As shown in FIG. 1A, a gate electrode 101 is formed over a substrate100. As the substrate 100, any of the following substrates can be used:non-alkaline glass substrates made of barium borosilicate glass,aluminoborosilicate glass, aluminosilicate glass, and the like by afusion method or a float method; ceramic substrates; plastic substrateshaving heat resistance enough to withstand a process temperature of thismanufacturing process; and the like. Alternatively, metal substrates ofa stainless-steel alloy and the like with the surface provided with aninsulating film may be employed. When the substrate 100 is mother glass,the substrate may have any of the following sizes: the first generation(320 mm×400 mm), the second generation (400 mm×500 mm), the thirdgeneration (550 mm×650 mm), the fourth generation (680 mm×880 mm, or 730mm×920 mm), the fifth generation (1000 mm×1200 mm, or 1100 mm×1250 mm),the sixth generation (1500 mm×1800 mm), the seventh generation (1900mm×2200 mm), the eighth generation (2160 mm×2460 mm), the ninthgeneration (2400 mm×2800 mm, or 2450 mm×3050 mm), the tenth generation(2950 mm×3400 mm), and the like.

The gate electrode 101 is formed using a metal material such astitanium, molybdenum, chromium, tantalum, tungsten, or aluminum; or analloy material thereof. The gate electrode 101 can be formed in such amanner that a conductive film is formed over the substrate 100 by asputtering method or a vacuum evaporation method; a mask is formed overthe conductive film by a photolithography technique or an inkjet method;and the conductive film is etched using the mask. Note that, as barriermetal which increases adhesion of the gate electrode 101 and preventsdiffusion to a base, a nitride film of the above-mentioned metalmaterial may be provided between the substrate 100 and the gateelectrode 101. Here, the gate electrode is formed by etching of theconductive film formed over the substrate 100 with the use of a resistmask formed using a first photomask.

Note that, because a semiconductor film and a wiring are to be formedover the gate electrode 101, it is desired that the gate electrode 101be processed so that its edge portions are tapered in order to preventdisconnection. Although not shown, in this step, a wiring connected tothe gate electrode can also be formed at the same time.

Next, over the gate electrode 101, a gate insulating film 102, amicrocrystalline semiconductor film 103, and a channel protection layer104 are formed in this order. Then, a resist 151 is applied on thechannel protection layer 104. Note that it is preferable that at leastthe gate insulating film 102, the microcrystalline semiconductor film103, and the channel protection layer 104 be formed successively. Bysuccessive formation of at least the gate insulating film 102, themicrocrystalline semiconductor film 103, and the channel protectionlayer 104 without any exposure to the atmosphere, each interface betweenstacked layers can be formed without being contaminated by anatmospheric constituent or a contaminant impurity element floating inthe atmosphere. Thus, variations in characteristics of thin filmtransistors can be reduced.

The gate insulating film 102 can be formed by a CVD method, a sputteringmethod, or the like using a silicon oxide film, a silicon nitride film,a silicon oxynitride film, or a silicon nitride oxide film. Note thatthe gate insulating film 102 can be formed not by a single layer but bytwo-stacked layers of a silicon oxide film or a silicon oxynitride film,and a silicon nitride film or a silicon nitride oxide film in thisorder. Note that the gate insulating film can be formed by stacking nottwo layers but three layers of a silicon nitride film or a siliconnitride oxide film, a silicon oxide film or a silicon oxynitride film,and a silicon nitride film or a silicon nitride oxide film in this orderfrom the substrate side.

Here, a silicon oxynitride film means a film that contains more oxygenthan nitrogen and includes oxygen, nitrogen, silicon, and hydrogen atconcentrations ranging from 55 at. % (atomic percent) to 65 at. %, 1 at.% to 20 at. %, 25 at. % to 35 at. %, and 0.1 at. % to 10 at. %,respectively. Further, a silicon nitride oxide film means a film thatcontains more nitrogen than oxygen and includes oxygen, nitrogen,silicon, and hydrogen at concentrations ranging from 15 at. % to 30 at.%, 20 at. % to 35 at. %, 25 at. % to 35 at. %, and 15 at. % to 25 at. %,respectively.

The microcrystalline semiconductor film 103 is a film which contains asemiconductor having an intermediate structure between amorphous andcrystalline structures (including a single crystal and a polycrystal).This semiconductor is a semiconductor which has a third state that isstable in terms of free energy, and is a crystalline semiconductor thathas short-range order and lattice distortion, and a columnar crystal orneedle crystal having crystal grains with a diameter of 0.5 nm to 20 nmis grown in the direction of the normal to a substrate surface. Inaddition, a microcrystalline semiconductor and a non-single crystalsemiconductor coexist. Microcrystalline silicon, which is a typicalexample of a microcrystalline semiconductor, has a Raman spectrum thatis shifted to a lower wave number side than 521 cm⁻¹ that representssingle-crystal silicon. That is, the peak of a Raman spectrum ofmicrocrystalline silicon is between 480 cm⁻¹ that shows amorphoussilicon and 521 cm⁻¹ that shows single-crystal silicon. In addition,microcrystalline silicon is made to contain hydrogen or halogen of atleast 1 at. % or more for termination of dangling bonds. Moreover,microcrystalline silicon is made to contain a noble gas element such ashelium, argon, krypton, neon, or the like to further enhance latticedistortion, whereby stability is increased and a favorablemicrocrystalline semiconductor film can be obtained. Such amicrocrystalline semiconductor film is disclosed in, for example, U.S.Pat. No. 4,409,134.

The microcrystalline semiconductor film can be formed by ahigh-frequency plasma CVD method with a frequency of several tens toseveral hundreds of megahertz or a microwave plasma CVD apparatus with afrequency of 1 GHz or more. The microcrystalline semiconductor film canbe typically formed using a dilution of silicon hydride such as SiH₄,Si₂H₆, or the like with hydrogen. With a dilution with one or pluralkinds of noble gas elements selected from helium, argon, krypton, andneon in addition to silicon hydride and hydrogen, the microcrystallinesemiconductor film can be formed. In that case, the flow ratio ofhydrogen to silicon hydride is set to be 50:1 to 1000:1, preferably,50:1 to 200:1, more preferably, 100:1. Note that, in place of siliconhydride, SiH₂Cl₂, SiHCl₃, SiCl₄, SiF₄, or the like can be used.

A microcrystalline semiconductor film exhibits weak n-type electricalconductivity when an impurity element for valence control is notintentionally added. Thus, threshold control of a microcrystallinesemiconductor film which functions as a channel formation region of athin film transistor can be achieved by addition of an impurity elementthat imparts p-type conductivity at the same time as or after the filmformation. A typical example of an impurity element that imparts p-typeconductivity is boron, and an impurity gas such as B₂H₆, BF₃, or thelike may be mixed into silicon hydride at a proportion of 1 ppm to 1000ppm, preferably, 1 ppm to 100 ppm. The concentration of boron may be setto be, for example, 1×10¹⁴ atoms/cm³ to 6×10¹⁶ atoms/cm³.

The oxygen concentration of the microcrystalline semiconductor film ispreferably set at 5×10¹⁹ cm⁻³ or less, more preferably, 1×10¹⁹ cm⁻³ orless, and each of the nitrogen concentration and the carbonconcentration is preferably set at 3×10¹⁸ cm³ or less. By decreases inconcentrations of oxygen, nitrogen, and carbon to be mixed into themicrocrystalline semiconductor film, the microcrystalline semiconductorfilm can be prevented from being changed into an n type.

The microcrystalline semiconductor film 103 is formed at a thickness of1 nm or more and 50 nm or less, preferably, 5 nm or more and 20 nm orless. The microcrystalline semiconductor film 103 functions as a channelformation region of a thin film transistor to be formed later. When thethickness of the microcrystalline semiconductor film 103 is within therange from 5 nm to 50 nm, a thin film transistor to be formed later isto be a fully depleted type. In addition, because the formation rate ofthe microcrystalline semiconductor film 103 is low, i.e., a tenth to ahundredth of that of an amorphous semiconductor film, a decrease inthickness leads to an increase in throughput. Furthermore, because themicrocrystalline semiconductor film contains microcrystals, it has lowerresistance than an amorphous semiconductor film. Therefore, a thin filmtransistor using the microcrystalline semiconductor film hascurrent-voltage characteristics represented by a curve with a steepslope in a rising portion, has an excellent response as a switchingelement, and can be operated at high speed. With the use of themicrocrystalline semiconductor film in a channel formation region of athin film transistor, fluctuation of a threshold voltage of a thin filmtransistor can be suppressed. Therefore, a display device with lessvariation of electrical characteristics can be manufactured.

The microcrystalline semiconductor film has higher mobility than anamorphous semiconductor film. Thus, with the use of a thin filmtransistor, a channel formation region of which is formed of themicrocrystalline semiconductor film, for switching of a liquid crystalelement that is a display element, the area of the channel formationregion, that is, the area of the thin film transistor can be decreased.Accordingly, the area of the thin film transistor in a single pixel isdecreased, and an aperture ratio of the pixel can be increased.

Note that, for an improvement in electric characteristics of themicrocrystalline semiconductor film, the gate insulating film may beirradiated with a laser beam from a surface side of the microcrystallinesemiconductor. The laser beam illuminates in the energy density thatmicrocrystalline semiconductor film does not melt. That is, laserprocess to the microcrystalline semiconductor film is performed bysolid-phase crystal growth without melting the microcrystallinesemiconductor film by radiant heating. In other words, it is to use acritical region in which the deposited microcrystalline semiconductorfilm does not become a liquid phase, and it can be referred to as“critical growth” in the meaning.

A laser beam can operate up to the interface between themicrocrystalline semiconductor film and the gate insulating film.Accordingly, when crystal in the surface side of the microcrystallinesemiconductor film is used as a seed, solid-phase crystal growthproceeds from the surface to the interface of the gate insulating film,and substantially columnar crystal grows. The solid-phase crystal growthby laser process does not enlarge the grain size but rather improves thecrystallinity in the direction of film thickness. A laser beam isconverged in a rectangular long shape (a linear laser beam), and laserprocess can be performed in such a way that a microcrystallinesemiconductor film over a glass substrate of 730 mm×920 mm is scanned byone laser beam, for example. In this case, a rate (overlap rate) foroverlapping a linear laser beam is set at 0% to 90% (preferably, 0% to67%). Thus, processing time for each substrate is shortened, andproductivity can be improved. The shape of a laser beam is not limitedto a linear shape, and a planar laser beam can also be used to performthe treatment similarly. In addition, this laser process is not limitedto the size of the above glass substrate and can be applied to varioussizes. Because of the laser process, the crystallinity in the interfaceregion of the gate insulating film is improved, and electriccharacteristics of a transistor having a bottom gate structure can beimproved. In such a critical growth, conventional unevenness (aprojecting body referred to as a ridge) at the surface which has beenseen in low temperature polycrystalline silicon is not formed and thesmoothness of the surface of the semiconductor film after laser processis kept. As in this embodiment mode, a crystalline semiconductor film,which is obtained in such a way that a microcrystalline semiconductorfilm after film formation is irradiated with a laser beam directly, isclearly different from the deposited microcrystalline semiconductor filmand a microcrystalline semiconductor film (described in Non-PatentDocument 1) which is improved by conduction heating, in the growthmechanism and film quality. In this specification, the microcrystallinesemiconductor film after film formation (semi amorphous semiconductor:SAS) which is subjected to laser process (hereinafter referred to as“LP”) is generally referred to as an LPSAS (laser process semi amorphoussemiconductor).

The channel protection layer 104 is formed of a silicon nitride film ora silicon nitride oxide film at a thickness of 400 nm or less,preferably, 50 nm or more and 200 nm or less. For example, the siliconnitride film is formed using SiH₄ and NH₃ as a source gas by a plasmaCVD method. The silicon nitride oxide film is formed using SiH₄, N₂O,and NH₃ by a plasma CVD method. Since the channel protection layer 104is provided in contact with the microcrystalline semiconductor film, thechannel protection layer 104 is formed of a silicon nitride film or asilicon nitride oxide film, whereby an effect of preventing diffusion ofimpurities to the microcrystalline semiconductor film can be obtainedand oxidation at the surface of crystal grains contained in themicrocrystalline semiconductor film can be prevented. Further, thechannel protection layer 104 is provided, whereby oxidation at thesurface of the microcrystalline semiconductor film can be prevented;therefore, the thickness of the microcrystalline semiconductor film canbe made small. Accordingly, since a thin film transistor in thisembodiment mode can be operated as a complete depletion type transistor,leakage current when the transistor is turned off can be reduced.

Here, a plasma CVD apparatus, with which from the gate insulating film102 to the channel protection layer 104 can be formed successively, isdescribed with reference to FIG. 6. FIG. 6 is a schematic diagramshowing an upper cross-sectional view of a plasma CVD apparatus, whichhas a structure where a loading chamber 1010, an unloading chamber 1015,and a reaction chamber (1) 1011, a reaction chamber (2) 1012, and areaction chamber (3) 1013 are provided around a common chamber 1020.Between the common chamber 1020 and the other chambers, gate valves1022, 1023, 1024, 1025, and 1026 are provided so that processesperformed in the chambers do not interface with each other. Substratesare loaded into a cassette 1028 in the loading chamber 1010 and acassette 1029 in the unloading chamber 1015 and carried to the reactionchambers (1) 1011 to (3) 1013 with a transport means 1021 of the commonchamber 1020. In this apparatus, a reaction chamber can be provided foreach kind of films to be deposited, and a plurality of different filmscan be formed successively without any exposure to the atmosphere. As anexample, it is possible to provide a structure in which the gateinsulating film 102 is formed in the reaction chamber (1) 1011, themicrocrystalline semiconductor film 103 is formed in the reactionchamber (2) 1012, and the channel protection layer 104 is formed in thereaction chamber (3) 1013.

In this manner, with a microwave plasma CVD apparatus where a pluralityof chambers is connected, the gate insulating film 102, themicrocrystalline semiconductor film 103, and the channel protectionlayer 104 can be formed at the same time. Thus, mass productivity can beimproved. In addition, even when maintenance or cleaning is performed inone of reaction chambers, film formation processes can be performed inthe other reaction chambers, whereby cycle time for film formation canbe shortened. Furthermore, each interface between stacked layers can beformed without being contaminated by an atmospheric constituent or acontaminant impurity element floating in the atmosphere. Thus,variations in characteristics of thin film transistors can be reduced.

Note that the plasma CVD apparatus shown in FIG. 6 is provided with theloading chamber and the unloading chamber separately, which may be asingle loading/unloading chamber. In addition, the plasma CVD apparatusmay be provided with a plurality of spare chambers. By preheating of asubstrate in the spare chamber, heating time needed before filmformation in each reaction chamber can be shortened; thus, throughputcan be improved.

FIG. 1A will be described again. For the resist 151 in FIG. 1A, apositive resist or a negative resist can be used. In this embodimentmode, a positive resist is used. Then, a second photomask is used, and aresist mask in which the resist 151 is processed as shown in FIG. 1A isformed. Then, as shown in FIG. 1B, the microcrystalline semiconductorfilm 103 and the channel protection layer 104 are etched by the resistmask formed over the channel protection layer, and an island-shapedmicrocrystalline semiconductor film 105 is formed over the gateelectrode 101. Note that FIG. 1B corresponds to a cross-sectional viewtaken along line A-B of FIG. 5A (however, the resist 151 and the gateinsulating film 102 are excluded). Note that, in this specification, anisland-shaped crystalline semiconductor film in which a microcrystallinesemiconductor film and a channel protection layer are stacked isdescribed. Note that a scan line 501 is shown in FIG. 5A, and the scanline 501 and the gate electrode 101 are electrically connected to eachother.

Note that the side surface of each end portion of the island-shapedmicrocrystalline semiconductor film 105 is tilted, whereby good electricconnection between an amorphous semiconductor film that is formed on theside surfaces of the island-shaped microcrystalline semiconductor filmand the microcrystalline semiconductor film at the bottom portion of theisland-shape microcrystalline semiconductor film 105 can be obtained.The angle of inclination of the side surface of each end portion of theisland-shaped microcrystalline semiconductor film 105 is set at 30° to90°, preferably, 45° to 80°. With such an angle, a disconnection of asource electrode or a drain electrode due to a step shape can beprevented.

Next, as shown in FIG. 1C, an amorphous semiconductor film 106 is formedso as to cover the island-shaped microcrystalline semiconductor film105, an impurity semiconductor layer 107 is formed over the amorphoussemiconductor film 106, and a conductive film 108 is formed over theimpurity semiconductor layer 107. Note that, when the shape of theconductive film 108 is processed, the conductive film 108 serves as asource electrode, a drain electrode, and a wiring of a signal line. Fora resist 152, a positive resist or a negative resist can be used. Inthis embodiment mode, a positive resist is used. A resist mask is formedusing a third photomask. As an example, wet etching which is isotropicetching is performed from a hole portion 171 formed in the resist mask,as shown in FIG. 2A in this embodiment mode. Wet etching is performed,whereby a hole portion 172 that is larger than the diameter of the holeportion 171 is formed as shown in FIG. 2B in the conductive film 108under the hole portion 171. Next, dry etching which is anisotropicetching is performed from the hole portion 171 formed in the resistmask. The dry etching is performed, whereby a hole portion 173 that hasalmost the same size as the hole portion 171 formed in the resist maskis formed in the impurity semiconductor layer 107 and the amorphoussemiconductor film 106 under the hole portion 171. As a result, as shownin FIG. 3A, the end portions of the conductive film 108 to serve as asource electrode and a drain electrode later and the end portions of theimpurity semiconductor layer 107 are not aligned with each other (awidth 174 in FIG. 3A), and the end portions of the impuritysemiconductor layer 107 are formed on an outer side of the end portionsof the conductive film 108. As shown in FIG. 3A, the end portions of theconductive film 108 to serve as a source electrode and a drain electrodelater and the end portions of the impurity semiconductor layer 107 havethe width 174 having a shape in which the end portions of the conductivefilm 108 and the end portions of the impurity semiconductor layer 107are not aligned with each other, whereby the distance of end portions ofthe source electrode and the drain electrode is increased; therefore,leakage current and a short circuit between the source electrode and thedrain electrode can be prevented. In addition, the end portions of theconductive film 108 to serve as a source electrode and a drain electrodelater and the end portions of the impurity semiconductor layer 107 havethe width 174 having a shape in which the end portions of the conductivefilm 108 and the end portions of the impurity semiconductor layer 107are not aligned with each other, whereby an electric field is notconcentrated on the end portions of the conductive film 108 and theimpurity semiconductor layer 107, and leakage current between the gateelectrode 101 and the conductive film 108 can be prevented. Accordingly,a thin film transistor with high reliability and high withstand voltagecan be formed. Then, the resist mask is removed, and an opening as shownin FIG. 3A can be obtained. Note that FIG. 3A corresponds to across-sectional view taken along line A-B of FIG. 5B (however, the gateinsulating film 102 is excluded). Note that a signal line 502, a sourceelectrode 108 a, and a drain electrode 108 b are shown in FIG. 5B, andthe signal line 502 and the source electrode 108 a are electricallyconnected to each other.

Note that it is difficult to define which one is referred to as a sourceelectrode or a drain electrode because a source electrode and a drainelectrode of a transistor are changed by operating conditions of thetransistor, and the like. Thus, in this embodiment mode, an electrodeconnected to the signal line 502 denotes the source electrode 108 a, andan electrode to be connected to a pixel electrode later denotes thedrain electrode 108 b.

Note that, as shown in FIG. 5B, it is found that the end portions of theimpurity semiconductor layer 107 are located outside the end portions ofthe source electrode 108 a and the drain electrode 108 b. In addition,one of the source electrode 108 a and the drain electrode 108 b has ashape that surrounds the other of the source electrode 108 a and thedrain electrode 108 b (specifically, U shape or C shape). Therefore,since the area of a region where carriers move can be increased, theamount of current can be increased and the area of the thin filmtransistor can be reduced. In addition, the microcrystallinesemiconductor film 103, the amorphous semiconductor film 106, theimpurity semiconductor layer 107, the source electrode 108 a, and thedrain electrode 108 b are superposed over the gate electrode 101;therefore, an effect of the unevenness of the gate electrode 101 issmall and reduction in coverage and generation of leakage current can besuppressed.

The thin film transistor described in this embodiment mode as shown inFIG. 3A is provided with an amorphous semiconductor film on the sidesurface of an island-shaped microcrystalline semiconductor film. Theamorphous semiconductor film has a thickness larger than themicrocrystalline semiconductor film that is provided in advance, wherebythe parasitic capacitance which is generated between a source electrodeand/or a drain electrode and a gate electrode can be reduced. Typically,the amorphous semiconductor film preferably has a thickness of 200 nm ormore and 400 nm or less. In addition, carriers (electrons and holes)flowing between the source electrode and the drain electrode of the thinfilm transistor flow between the source electrode and the drainelectrode via the microcrystalline semiconductor film that forms aninterface with the gate insulating film near the gate electrode. Thethin film transistor has a longer distance in the direction of thethickness of the amorphous semiconductor film where carriers flow than adistance in a channel length direction of the microcrystallinesemiconductor film where carriers flow. Accordingly, in a display deviceprovided with a thin film transistor of the present invention, parasiticcapacitance which is generated between the source electrode and/or thedrain electrode and the gate electrode can be reduced while a good pointof the microcrystalline semiconductor film is utilized. In addition, ina display device that has high applied voltage to a gate electrode(e.g., around 15 V), when the thickness of the amorphous semiconductorfilm is larger than that of the microcrystalline semiconductor film,withstand voltage between a gate electrode and a source electrode and/ora drain electrode is increased and deterioration of a thin filmtransistor can be suppressed.

The amorphous semiconductor film 106 can be formed by a plasma CVDmethod using silicon hydride such as SiH₄, Si₂H₆, or the like.Alternatively, with a dilution of silicon hydride mentioned above withone or plural kinds of noble gas elements selected from helium, argon,krypton, and neon, an amorphous semiconductor film can be formed. Withthe use of hydrogen at a flow rate which is 1 to 20 times, preferably, 1to 10 times, more preferably, 1 to 5 times higher than that of siliconhydride, a hydrogen-containing amorphous semiconductor film can beformed. With the use of silicon hydride mentioned above and nitrogen orammonia, a nitrogen-containing amorphous semiconductor film can beformed. With the use of silicon hydride mentioned above and a gascontaining fluorine, chlorine, bromine, or iodine (F₂, Cl₂, Br₂, I₂, HF,HCl, HBr, HI, or the like), an amorphous semiconductor film containingfluorine, chlorine, bromine, or iodine can be formed. Note that, inplace of silicon hydride, SiH₂Cl₂, SiHCl₃, SiCl₄, SiF₄, or the like canbe used.

The amorphous semiconductor film 106 has a larger energy gap than themicrocrystalline semiconductor film 103 (the energy gap of the amorphoussemiconductor film is 1.6 eV to 1.8 eV and the energy gap of themicrocrystalline semiconductor film is 1.1 eV to 1.5 eV) and has higherresistance, and has lower mobility, i.e., a fifth to a tenth of that ofthe microcrystalline semiconductor film. Therefore, in a thin filmtransistor to be formed later, although part of the amorphoussemiconductor film 106 formed between source and drain regions and themicrocrystalline semiconductor film functions as a channel formationregion, most of the amorphous semiconductor film 106 functions as ahigh-resistance region and the microcrystalline semiconductor filmfunctions as a channel formation region. Accordingly, the off-current ofthe thin film transistor can be reduced.

In the case where an n-channel thin film transistor is to be formed, tothe impurity semiconductor layer 107 to which an impurity imparting oneconductivity type is added, phosphorus may be added as a typicalimpurity element, and an impurity gas such as PH₃ may be added tosilicon hydride. In the case where a p-channel thin film transistor isto be formed, boron may be added as a typical impurity element, and animpurity gas such as B₂H₆ or the like may be added to silicon hydride.The impurity semiconductor layer 107 to which an impurity imparting oneconductivity type is added can be formed of a microcrystallinesemiconductor or an amorphous semiconductor. Further, the impuritysemiconductor layer 107 to which an impurity imparting one conductivitytype is added may be formed using a stacked layer of an amorphoussemiconductor film to which an impurity imparting one conductivity typeis added and a microcrystalline semiconductor film to which an impurityimparting one conductivity type is added. The impurity semiconductorlayer 107 to which an impurity imparting one conductivity type is addedis formed at a thickness of 2 nm or more and 50 nm or less. By formationof the semiconductor film to which an impurity imparting oneconductivity type is added to a small thickness, throughput can beimproved.

It is preferable that the conductive film 108 be formed using a singlelayer or a stacked layer of aluminum, or a single layer or a stackedlayer of an aluminum alloy to which an element to improve heatresistance or an element to prevent a hillock such as copper, silicon,titanium, neodymium, scandium, or molybdenum is added. Alternatively,the conductive film may have a stacked-layer structure where a film onthe side in contact with the conductive semiconductor film is formed oftitanium, tantalum, molybdenum, tungsten, or nitride of any of theseelements and an aluminum film or an aluminum alloy film is formedthereover. Still alternatively, the conductive film may have astacked-layer structure where an aluminum film or an aluminum alloy filmis sandwiched between upper and lower films of titanium, tantalum,molybdenum, tungsten, or nitride of any of these elements. Here, as theconductive film 108, a conductive film in which three conductive filmsare stacked is given. A stacked-layer conductive film where an aluminumfilm is sandwiched between molybdenum films and a stacked-layerconductive film where an aluminum film is sandwiched between titaniumfilms can be given as examples. The conductive film is formed by asputtering method or a vacuum evaporation method.

Note that an impurity element for valence control with respect to theabove-described microcrystalline semiconductor film may be added so asto perform doping through the channel protection layer 104 after anetching process is performed on the amorphous semiconductor film 106,the impurity semiconductor layer 107, and the conductive film 108 overthe microcrystalline semiconductor film. After an etching process isperformed on the amorphous semiconductor film 106, the impuritysemiconductor layer 107, and the conductive film 108 over themicrocrystalline semiconductor film, an impurity element can beselectively added to the island-shaped microcrystalline semiconductorfilm 105 to serve as a channel formation region by performing dopingthrough the channel protection layer 104.

Through the above-described steps, a thin film transistor can be formed.In addition, a thin film transistor can be formed using three pieces ofphotomasks.

Next, as shown in FIG. 3B, an insulating film 109 is formed over theconductive film 108, the impurity semiconductor layer 107, the amorphoussemiconductor film 106, the island-shaped microcrystalline semiconductorfilm 105, and the gate insulating film 102. The insulating film 109 canbe formed in a similar manner to the gate insulating film 102. Note thatthe insulating film 109 is provided to prevent entry of a contaminantimpurity such as an organic substance, a metal substance, or moisturefloating in the atmosphere and is preferably a dense film.

Next, as shown in FIG. 3C, a contact hole 110 is formed in theinsulating film 109. Then, a pixel electrode 111 is formed in thecontact hole 110 to be in contact with the drain electrode 108 b of theconductive film 108, as shown in FIG. 4. Note that FIG. 4 corresponds toa cross-sectional view taken along line A-B of FIG. 5C.

The pixel electrode 111 can be formed using a light-transmittingconductive material such as indium oxide containing tungsten oxide,indium zinc oxide containing tungsten oxide, indium oxide containingtitanium oxide, indium tin oxide containing titanium oxide, indium tinoxide (hereinafter referred to as ITO), indium zinc oxide, or indium tinoxide to which silicon oxide is added.

Alternatively, the pixel electrode 111 can be formed using a conductivecomposition containing a conductive high-molecular compound (alsoreferred to as a conductive polymer). It is preferable that the pixelelectrode formed using the conductive composition have a sheetresistance of 10,000 Ω/square or less and a light transmittance of 70%or more at a wavelength of 550 nm. In addition, it is preferable thatthe resistivity of the conductive high-molecular compound contained inthe conductive composition be 0.1Ω·cm or less.

As the conductive high-molecular compound, a so-called π electronconjugated conductive high-molecular compound can be used. Examplesinclude polyaniline and its derivatives, polypyrrole and itsderivatives, polythiophene and its derivatives, copolymers of two ormore kinds of them, and the like.

As described above, a thin film transistor which can be used for adisplay device can be obtained. In particular, in a thin film transistorobtained by this embodiment mode suppresses, an increase in parasiticcapacitance and an increase in production cost can be suppressed while adecrease in yield can be suppressed, and high electric characteristicsand reduction in off-current can be achieved. Therefore, a displaydevice that is driven by a thin film transistor having high reliabilityof electric characteristics can be obtained.

This embodiment mode can be implemented in combination with any of thestructures described in the other embodiment modes, as appropriate.

Embodiment Mode 2

In this embodiment mode, a thin film transistor used for a displaydevice, which is different from that described in Embodiment Mode 1,will be described with reference to FIG. 13. FIG. 13 is across-sectional view of a thin film transistor. Note that, in thisembodiment mode, portions that are similar to the portions in EmbodimentMode 1 are denoted by the same reference numerals and description ismade below with reference to the description of Embodiment Mode 1.

Note that the thin film transistor described in this embodiment mode isan n-channel thin film transistor similarly to the case of EmbodimentMode 1.

First, the gate electrode 101, the gate insulating film 102, themicrocrystalline semiconductor film 103, the channel protection layer104 are formed over the substrate 100, and a state shown in FIG. 1B ofEmbodiment Mode 1 is obtained by a resist mask and an etching process.The substrate 100, the gate electrode 101, the gate insulating film 102,the microcrystalline semiconductor film 103, and the channel protectionlayer 104 are the same as those used in Embodiment Mode 1.

Next, as shown in FIG. 13, an amorphous semiconductor film 1301 acontaining an impurity element that imparts weak p-type conductivity,and an amorphous semiconductor film 1301 b to serve as an intrinsicsemiconductor are formed. The impurity semiconductor layer 107 and theconductive film 108 are formed over the amorphous semiconductor film1301 b to serve as an intrinsic semiconductor in a similar way to thatof Embodiment Mode 1. Note that the amorphous semiconductor film 1301 acontaining an impurity element that imparts weak p-type conductivity andthe amorphous semiconductor film 1301 b to serve as an intrinsicsemiconductor are formed, and then the impurity semiconductor layer 107and the conductive film 108 are formed in a similar manner to theamorphous semiconductor film 106 described in Embodiment Mode 1, and aresist mask is formed and an etching process is performed. Then, a thinfilm transistor as shown in FIG. 13 can be obtained. In addition, theobtained thin film transistor is provided with the insulating film 109so as to cover the thin film transistor in a similar manner to that inEmbodiment Mode 1 and can be electrically connected to the pixelelectrode 111 by the contact hole 110.

Electric carriers flowing between a source electrode and a drainelectrode of the thin film transistor described in this embodiment modeflow through the conductive film 108 (a source electrode or a drainelectrode), the impurity semiconductor layer 107, the amorphoussemiconductor film 1301 b to serve as an intrinsic semiconductor, theamorphous semiconductor film 1301 a containing an impurity element thatimparts weak p-type conductivity, the microcrystalline semiconductorfilm 103, the amorphous semiconductor film 1301 a containing an impurityelement that imparts weak p-type conductivity, the amorphoussemiconductor film 1301 b to serve as an intrinsic semiconductor, theimpurity semiconductor layer 107, and the conductive film 108 (thesource electrode or the drain electrode) in this order. That is, theelectric carriers flowing between the source electrode and the drainelectrode of the thin film transistor described in this embodiment modepass through the amorphous semiconductor film 1301 a containing animpurity element that imparts weak p-type conductivity to serve as ahigh-resistance region and the amorphous semiconductor film 1301 b toserve as an intrinsic semiconductor. Accordingly, the thin filmtransistor described in this embodiment mode can reduce leakage currentflowing between the source electrode and the drain electrode. Therefore,the thin film transistor described in this embodiment mode can have aneffect of reducing leakage current as well as superior electriccharacteristics described in Embodiment Mode 1.

This embodiment mode can be implemented in combination with any of thestructures described in the other embodiment modes, as appropriate.

Embodiment Mode 3

In this embodiment mode, a thin film transistor used for a displaydevice, which is different from those described in Embodiment Mode 1 andEmbodiment Mode 2, will be described with reference to FIG. 14. FIG. 14is a cross-sectional view of a thin film transistor. Note that, in thisembodiment mode, portions that are similar to the portions in EmbodimentMode 1 are denoted by the same reference numerals and description ismade below with reference to the description of Embodiment Mode 1.

Note that the thin film transistor described in this embodiment mode isan n-channel thin film transistor, similarly to the case of EmbodimentMode 1.

First, the gate electrode 101, the gate insulating film 102, themicrocrystalline semiconductor film 103, the channel protection layer104 are formed over the substrate 100, and a state shown in FIG. 1B ofEmbodiment Mode 1 is obtained by a resist mask and an etching process.The substrate 100, the gate electrode 101, the gate insulating film 102,the microcrystalline semiconductor film 103, and the channel protectionlayer 104 are the same as those used in Embodiment Mode 1.

Next, as shown in FIG. 14, an amorphous semiconductor film 1401 a toserve as an intrinsic semiconductor and an amorphous semiconductor film1401 b containing an impurity element that imparts weak n-typeconductivity, are formed. The impurity semiconductor layer 107 and theconductive film 108 are formed over the amorphous semiconductor film1401 b containing an impurity element that imparts weak n-typeconductivity in a similar way to that of Embodiment Mode 1. Note thatthe amorphous semiconductor film 1401 a to serve as an intrinsicsemiconductor and the amorphous semiconductor film 1401 b containing animpurity element that imparts weak n-type conductivity are formed, andthen the impurity semiconductor layer 107 and the conductive film 108are formed in a similar manner to the amorphous semiconductor film 106described in Embodiment Mode 1, and a resist mask is formed and anetching process is performed. Then, a thin film transistor as shown inFIG. 14 can be obtained. In addition, the obtained thin film transistoris provided with the insulating film 109 so as to cover the thin filmtransistor in a similar manner to that in Embodiment Mode 1 and can beelectrically connected to the pixel electrode 111 by the contact hole110.

Electric carriers flowing between a source electrode and a drainelectrode of the thin film transistor described in this embodiment modeflow through the conductive film 108 (the source electrode or the drainelectrode), the impurity semiconductor layer 107, the amorphoussemiconductor film 1401 b containing an impurity element that impartsweak n-type conductivity, the amorphous semiconductor film 1401 a toserve as an intrinsic semiconductor, the microcrystalline semiconductorfilm 103, the amorphous semiconductor film 1401 a to serve as anintrinsic semiconductor, the amorphous semiconductor film 1401 bcontaining an impurity element that imparts weak n-type conductivity,the impurity semiconductor layer 107, and the conductive film 108 (thesource electrode or the drain electrode) in this order. That is, for theelectric carriers flowing between the source electrode and the drainelectrode of the thin film transistor described in this embodiment mode,leakage current can be decreased because semiconductor films are stackedso as to have high-resistance regions in which resistance graduallyincreases from the impurity semiconductor layer 107, the amorphoussemiconductor film 1401 b containing an impurity element that impartsweak n-type conductivity, and the amorphous semiconductor film 1401 a toserve as an intrinsic semiconductor; and deterioration of the thin filmtransistor due to electrons accelerated by a sudden change of voltagebecause a resistance value gradually increases can be reduced.Accordingly, in this embodiment mode, leakage current flowing betweenthe source electrode and the drain electrode can be reduced, and thelife of the thin film transistor can be increased. Therefore, the thinfilm transistor described in this embodiment mode can have an effect ofreducing leakage current as well as superior electric characteristicsdescribed in Embodiment Mode 1.

This embodiment mode can be implemented in combination with any of thestructures described in the other embodiment modes, as appropriate.

Embodiment Mode 4

In this embodiment mode, a display device having the thin filmtransistor described in Embodiment Mode 1 will be described below. Thedisplay device described in this embodiment mode is described using aliquid crystal display device as an example.

An external view and a cross section of a liquid crystal display panelwhich is one mode of the liquid crystal display device will be describedwith reference to FIGS. 7A and 7B. FIG. 7A is a top view of the panel inwhich a thin film transistor 4010 including a microcrystallinesemiconductor film and a liquid crystal element 4013 formed over a firstsubstrate 4001 are sealed between a second substrate 4006 and the firstsubstrate 4001 with a sealant 4005, and FIG. 7B is a cross-sectionalview taken along line M-N of FIG. 7A.

The sealant 4005 is provided so as to surround a pixel portion 4002 anda scan line driver circuit 4004 which are provided over the firstsubstrate 4001. The second substrate 4006 is provided over the pixelportion 4002 and the scan line driver circuit 4004. Therefore, the pixelportion 4002 and the scan line driver circuit 4004 are sealed, togetherwith liquid crystal 4008, between the first substrate 4001 and thesecond substrate 4006 with the sealant 4005. A signal line drivercircuit 4003 that is formed using a polycrystalline semiconductor filmover a separately prepared substrate is mounted at a region that isdifferent from the region surrounded by the sealant 4005 over the firstsubstrate 4001. In this embodiment mode, an example of attaching thesignal line driver circuit including a thin film transistor formed usinga polycrystalline semiconductor film to the first substrate 4001 will bedescribed. Alternatively, a signal line driver circuit including atransistor, which is formed using a single-crystalline semiconductor,may be attached to the first substrate 4001. FIG. 7B exemplifies a thinfilm transistor 4009 formed using a polycrystalline semiconductor film,which is included in the signal line driver circuit 4003.

The pixel portion 4002 and the scan line driver circuit 4004 formed overthe first substrate 4001 each include a plurality of thin filmtransistors, and the thin film transistor 4010 included in the pixelportion 4002 is illustrated as an example in FIG. 7B. The thin filmtransistor 4010 corresponds to a thin film transistor which uses amicrocrystalline semiconductor film and can be formed in a similarmanner to the process described in Embodiment Mode 1.

In addition, a pixel electrode 4030 facing the liquid crystal 4008 iselectrically connected to the thin film transistor 4010 through a wiring4040. A counter electrode 4031 of the liquid crystal element 4013 isformed on the second substrate 4006. The liquid crystal element 4013corresponds to a region where the pixel electrode 4030 and the counterelectrode 4031 sandwich the liquid crystal 4008.

Note that, as the first substrate 4001 and the second substrate 4006,glass, metal (typically, stainless steel), ceramics, or plastic can beused. As for plastic, an FRP (fiberglass-reinforced plastics) plate, aPVF (polyvinyl fluoride) film, a polyester film, a polyester film, or anacrylic resin film can be used. In addition, a sheet with a structure inwhich an aluminum foil is sandwiched between PVF films or polyesterfilms can be used.

In addition, reference numeral 4035 is a spherical spacer which isprovided to control the distance (a cell gap) between the pixelelectrode 4030 and the counter electrode 4031. Note that a spacer whichis obtained by selectively etching an insulating film may be used.

A variety of signals and potential are supplied to the signal linedriver circuit 4003 which is separately formed and the scan line drivercircuit 4004 or the pixel portion 4002 via wirings 4014 and 4015 from anFPC 4018.

In this embodiment mode, a connection terminal 4016 is formed of thesame conductive film as that of the pixel electrode 4030 included in theliquid crystal element 4013. In addition, the wirings 4014 and 4015 areformed of the same conductive film as that of the wiring 4040.

The connection terminal 4016 is electrically connected to a terminal ofthe FPC 4018 via an anisotropic conductive film 4019.

Although not illustrated, the liquid crystal display device described inthis embodiment mode includes an alignment film, a polarizing plate, andfurther, may include a color filter and a light-blocking film.

Note that FIGS. 7A and 7B illustrate an example in which the signal linedriver circuit 4003 is separately formed and mounted on the firstsubstrate 4001, but this embodiment mode is not limited to thisstructure. The scan line driver circuit may be separately formed andthen mounted, or only part of the signal line driver circuit or part ofthe scan line driver circuit may be separately formed and then mounted.

This embodiment mode can be implemented in combination with any of thestructures described in the other embodiment modes, as appropriate.

Embodiment Mode 5

In this embodiment mode, a display device having the thin filmtransistor described in Embodiment Mode 1 will be described below. Thedisplay device described in this embodiment mode is described using alight-emitting device as an example.

An external view and a cross section of a light-emitting display panelwhich is one mode of the light-emitting device will be described withreference to FIGS. 8A and 8B. FIG. 8A is a top view of a panel in whicha thin film transistor and a light-emitting element which are formedover a first substrate using a microcrystalline semiconductor film aresealed between the first substrate and a second substrate with asealant, and FIG. 8B corresponds to a cross-sectional view taken alongline E-F of FIG. 8A.

A sealant 4505 is provided so as to surround a pixel portion 4502 and ascan line driver circuit 4504 which are provided over a first substrate4501. A second substrate 4506 is provided over the pixel portion 4502and the scan line driver circuit 4504. Therefore, the pixel portion 4502and the scan line driver circuit 4504 as well as a filler 4507 aresealed between the first substrate 4501 and the second substrate 4506with the sealant 4505. A signal line driver circuit 4503 that is formedusing a polycrystalline semiconductor film over a separately preparedsubstrate is mounted at a region that is different from the regionsurrounded by the sealant 4505 over the first substrate 4501. Thisembodiment mode will describe an example of attaching the signal linedriver circuit including a thin film transistor formed using apolycrystalline semiconductor film to the first substrate 4501.Alternatively, a signal line driver circuit including a transistor,which is formed using a single-crystalline semiconductor film, may beattached to the first substrate 4501. FIG. 8B exemplifies a thin filmtransistor 4509 formed using a polycrystalline semiconductor film, whichis included in the signal line driver circuit 4503.

The pixel portion 4502 and the scan line driver circuit 4504 formed overthe first substrate 4501 each include a plurality of thin filmtransistors, and a thin film transistor 4510 included in the pixelportion 4502 is illustrated as an example in FIG. 8B. In this embodimentmode, the thin film transistor 4510 is illustrated as a driving TFT butmay also be a current control TFT or an erasing TFT. The thin filmtransistor 4510 corresponds to a thin film transistor which uses amicrocrystalline semiconductor film and can be formed in a similarmanner to the process described in Embodiment Mode 1.

In addition, reference numeral 4511 corresponds to a light-emittingelement, and a pixel electrode of the light-emitting element 4511 iselectrically connected to a source electrode or a drain electrode of thethin film transistor 4510 via a wiring 4517. In this embodiment mode, acommon electrode of the light-emitting element 4511 and alight-transmitting conductive material 4512 are electrically connectedto each other. Note that a structure of the light emitting element 4511is not limited to the structure shown in the present embodiment mode.The structure of the light-emitting element 4511 can be changed asappropriate in accordance with a direction of light taken from thelight-emitting element 4511, polarity of the thin film transistor 4510,or the like.

Although a variety of signal's and potential which are applied to thesignal line driver circuit 4503 which is separately formed and the scanline driver circuit 4504 or the pixel portion 4502 are not illustratedin the cross-sectional view of FIG. 8B, the variety of signals and thepotential are supplied from an FPC 4518 via wirings 4514 and 4515.

In this embodiment mode, a connection terminal 4516 is formed of thesame conductive film as that of the pixel electrode included in thelight-emitting element 4511. In addition, the wirings 4514 and 4515 areformed of the same conductive film as that of the wiring 4517.

The connection terminal 4516 is electrically connected to a terminalincluded in the FPC 4518 through an anisotropic conductive film 4519.

The second substrate in a direction to extract light from thelight-emitting element 4511 needs to be transparent. In that case, alight-transmitting material such as a glass plate, a plastic plate, apolyester film, or an acrylic film is used.

As the filler 4507, an inert gas such as nitrogen or argon can be usedas well as an ultraviolet curable resin or a heat curable resin such asPVC (polyvinyl chloride), acrylic, polyimide, an epoxy resin, a siliconeresin, PVB (polyvinyl butyral), or EVA (ethylene vinyl acetate). In thisembodiment mode, nitrogen is used as a filler.

In addition, if needed, optical films, such as a polarizer, a circularpolarizer (including an elliptical polarizer), a retardation plate (aquarter-wave plate or a half-wave plate), a color filter, and the like,may be provided on a light-emitting surface of the light-emittingelement, as appropriate. Further, a polarizing plate or a circularlypolarizing plate may be provided with an anti-reflection film. Forexample, an anti-glare treatment which can diffuse reflected light witha depression and a projection on the surface, and reduce glare can beperformed.

Note that FIGS. 8A and 8B illustrate an example in which the signal linedriver circuit 4503 is separately formed and mounted on the firstsubstrate 4501, but this embodiment mode is not limited to thisstructure. The scan line driver circuit may be separately formed andthen mounted, or only part of the signal line driver circuit or part ofthe scan line driver circuit may be separately formed and then mounted.

This embodiment mode can be implemented in combination with any of thestructures described in the other embodiment modes, as appropriate.

Embodiment Mode 6

A structure of a display panel, which is one mode of a display device ofthe present invention, will be described below.

FIG. 9A shows a mode of a display panel in which a signal line drivercircuit 6013 which is separately formed is connected to a pixel portion6012 formed over a substrate 6011. The pixel portion 6012 and a scanline driver circuit 6014 are each formed using a thin film transistor inwhich a microcrystalline semiconductor film is used. When the signalline driver circuit is formed using a transistor in which highermobility can be obtained compared with the thin film transistor in whichthe microcrystalline semiconductor film is used, an operation of thesignal line driver circuit which demands higher driving frequency thanthat of the scan line driver circuit can be stabilized. Note that thesignal line driver circuit 6013 may be formed using a transistor using asingle crystalline semiconductor, a thin film transistor using apolycrystalline semiconductor, or a transistor that is formed using anSOI substrate. The pixel portion 6012, the signal line driver circuit6013, and the scan line driver circuit 6014 are each supplied with apotential of a power supply, a variety of signals, and the like via anFPC 6015.

Note that both the signal line driver circuit and the scan line drivercircuit may be formed over the same substrate as the pixel portion.

When a driver circuit is separately formed, a substrate over which thedriver circuit is formed is not necessarily attached to a substrate overwhich a pixel portion is formed, and may be attached over an FPC, forexample. FIG. 9B shows a mode of a liquid crystal display panel in whicha signal line driver circuit 6023 which is separately formed isconnected to a pixel portion 6022 and a scan line driver circuit 6024formed over a substrate 6021. The pixel portion 6022 and the scan linedriver circuit 6024 are each formed using a thin film transistor inwhich a microcrystalline semiconductor film is used. The signal linedriver circuit 6023 is connected to the pixel portion 6022 via an FPC6025. The pixel portion 6022, the signal line driver circuit 6023, andthe scan line driver circuit 6024 are each supplied with a potential ofa power supply, a variety of signals, and the like via the FPC 6025.

Alternatively, only part of a signal line driver circuit or part of ascan line driver circuit may be formed over the same substrate as apixel portion by using a thin film transistor in which amicrocrystalline semiconductor film is used, and the other part of thedriver circuit may be separately formed and electrically connected tothe pixel portion. FIG. 9C shows a mode of a liquid crystal displaypanel in which an analog switch 6033 a included in a signal line drivercircuit is formed over a substrate 6031, which is the same substrate asa pixel portion 6032 and a scan line driver circuit 6034, and a shiftregister 6033 b included in the signal line driver circuit is separatelyformed over a different substrate and attached to the substrate 6031.The pixel portion 6032 and the scan line driver circuit 6034 are eachformed using a thin film transistor in which a microcrystallinesemiconductor film is used. The shift register 6033 b included in thesignal line driver circuit is connected to the pixel portion 6032 via anFPC 6035. The pixel portion 6032, the signal line driver circuit, andthe scan line driver circuit 6034 are each supplied with a potential ofa power supply, a variety of signals, and the like via the FPC 6035.

As shown in FIGS. 9A to 9C, in a display device of the presentinvention, all or a part of the driver circuit can be formed over thesame substrate as the pixel portion, using the thin film transistor inwhich the microcrystalline semiconductor film is used.

Note that there is no particular limitation on a connection method of asubstrate which is separately formed, and a known COG method, wirebonding method, TAB method, or the like can be used. Further, aconnection position is not limited to the positions shown in FIGS. 9A to9C as long as electrical connection is possible. Moreover, a controller,a CPU, a memory, or the like may be separately formed and connected.

Note that a signal line driver circuit used in the present invention isnot limited to a structure including only a shift register and an analogswitch. In addition to the shift register and the analog switch, anothercircuit such as a buffer, a level shifter, a source follower, or thelike may be included. Moreover, the shift register and the analog switchare not necessarily provided. For example, a different circuit such as adecoder circuit by which a signal line can be selected may be usedinstead of the shift register, or a latch or the like may be usedinstead of the analog switch.

FIG. 10 is a block diagram of a liquid crystal display device of thepresent invention. The liquid crystal display device shown in FIG. 10includes a pixel portion 551 including a plurality of pixels eachprovided with a liquid crystal element, a scan line driver circuit 552which selects each pixel, and a signal line driver circuit 553 whichcontrols input of a video signal to a selected pixel.

In FIG. 10, the signal line driver circuit 553 includes a shift register554 and an analog switch 555. A clock signal (CLK) and a start pulsesignal (SP) are input to the shift register 554. When the clock signal(CLK) and the start pulse signal (SP) are input, a timing signal isgenerated in the shift register 554 and input to the analog switch 555.

A video signal is supplied to the analog switch 555. The analog switch555 samples the video signal in accordance with the input timing signaland supplies the resulting signal to a signal line of the next stage.

Next, a structure of the scan line driver circuit 552 is described. Thescan line driver circuit 552 includes a shift register 556 and a buffer557. The scan line driver circuit 552 may also include a level shifterin some cases. In the scan line driver circuit 552, when the clocksignal (CLK) and the start pulse signal (SP) are input to the shiftregister 556, a selection signal is generated. The generated selectionsignal is buffered and amplified by the buffer 557, and the resultingsignal is supplied to a corresponding scan line. Gates of transistors inpixels of one line are connected to the scan line. Further, since thetransistors in the pixels of one line have to be turned on at the sametime, a buffer through which large current can flow is used as thebuffer 557.

In a full color liquid crystal display device, when video signalscorresponding to R (red), G (green), or B (blue) are sequentiallysampled and supplied to a corresponding signal line, the number ofterminals for connecting the shift register 554 and the analog switch555 corresponds to approximately a third of the number of terminals forconnecting the analog switch 555 and the signal line in the pixelportion 551. Accordingly, when the analog switch 555 and the pixelportion 551 are formed over the same substrate, the number of terminalsused for connecting substrates which are separately formed can besuppressed compared with the case where the analog switch 555 and thepixel portion 551 are formed over different substrates; thus, occurrenceprobability of bad connection can be suppressed, and yield can beincreased.

Note that, although the scan line driver circuit 552 shown in FIG. 10includes the shift register 556 and the buffer 557, the scan line drivercircuit 552 may be formed using the shift register 556.

Note that structures of the signal line driver circuit and the scan linedriver circuit are not limited to the structures shown in FIG. 10, whichare merely one mode of the display device of the present invention.

This embodiment mode can be implemented in combination with any of thestructures described in the other embodiment modes.

Embodiment Mode 7

The display device obtained by the present invention can be used for anactive matrix liquid crystal module. That is, the present invention canbe implemented in any of electronic devices having a display portioninto which such an active matrix liquid crystal module is incorporated.

Examples of such electronic devices include cameras such as a videocamera and a digital camera, a head-mounted display (a goggle-typedisplay), a car navigation system, a projector, a car stereo, a personalcomputer, and a portable information terminal (e.g., a mobile computer,a cellular phone, and an e-book reader). FIGS. 11A to 11C show examplesof such electronic devices.

FIG. 11A shows a television device. The television device can becompleted by incorporating a display module into a housing, as shown inFIG. 11A. A display panel at the stage after an FPC is attached is alsoreferred to as a display module. A main screen 2003 is formed using thedisplay module, and other accessories such as a speaker portion 2009, anoperation switch, and the like are provided. Thus, the television devicecan be completed.

As shown in FIG. 11A, a display panel 2002 using a liquid crystalelement is incorporated into a housing 2001. The television device canreceive general TV broadcast by a receiver 2005, and can be connected toa wired or wireless communication network via a modem 2004 so thatone-way (from a sender to a receiver) or two-way (between a sender and areceiver or between receivers) information communication can beperformed. The television device can be operated by a switchincorporated into the housing or a separate remote control unit 2006.The remote control unit may include a display portion 2007 fordisplaying information to be output.

Further, the television device may include a sub screen 2008 formedusing a second display panel for displaying channels, sound volume, andthe like, in addition to the main screen 2003. In this structure, themain screen 2003 may be formed using a liquid crystal display panel withan excellent viewing angle, and the sub screen may be formed using aliquid crystal display panel in which display is performed with lowpower consumption. Alternatively, when reduction in power consumption isprioritized, a structure may be employed in which the main screen 2003is formed using a liquid crystal display panel, the sub screen is formedusing a liquid crystal display panel, and the sub screen can be turnedon and off.

FIG. 12 is a block diagram of a main structure of a television device. Adisplay panel 900 is provided with a pixel portion 921. A signal linedriver circuit 922 and a scan line driver circuit 923 may be mounted onthe display panel 900 by a COG method.

As for other external circuits, the television device includes a videosignal amplifier circuit 925 which amplifies a video signal amongsignals received by a tuner 924; a video signal processing circuit 926which converts a signal output from the video signal amplifier circuit925 into a color signal corresponding to each color of red, green, andblue; a control circuit 927 which converts the video signal into aninput specification of a driver IC; and the like on an input side of thevideo signal. The control circuit 927 outputs signals to each of thescan line side and the signal line side. When digital driving isperformed, a structure may be employed in which a signal dividingcircuit 928 is provided on the signal line side and an input digitalsignal is divided into m signals to be supplied.

Among the signals received by the tuner 924, an audio signal istransmitted to an audio signal amplifier circuit 929, and an outputthereof is supplied to a speaker 933 through an audio signal processingcircuit 930. A control circuit 931 receives control information onreceiving station (receiving frequency) and volume from an input portion932 and transmits a signal to the tuner 924 and the audio signalprocessing circuit 930.

It is needless to say that the present invention is not limited to atelevision device and can be applied to various uses, e.g., a monitor ofa computer, a large display medium such as an information display boardat the train station, the airport, or the like, or an advertisementdisplay board on the street, and the like.

FIG. 11B shows an example of a cellular phone 2301. The cellular phone2301 includes a display portion 2302, an operation portion 2303, and thelike. When the display device described in the above-describedembodiment mode is used for the display portion 2302, mass productivitycan be improved.

A portable computer shown in FIG. 11C includes a main body 2401, adisplay portion 2402, and the like. When the display device described inthe above-described embodiment mode is used for the display portion2402, mass productivity can be improved.

This embodiment mode can be implemented in combination with any of thestructures described in the other embodiment modes.

Embodiment Mode 8

In this embodiment mode, device simulation results of a transistorstructure of the present invention described in the above embodimentmode are shown. A transistor structure used for the device simulation isshown in FIG. 15, and current-voltage characteristics of the transistorstructure shown in FIG. 15 are shown in FIG. 16. Note that “ATLAS” madeby Silvaco Data Systems, Inc. is used for the device simulation.

A stacked structure of the transistor structure shown in FIG. 15 isdescribed. The structure shown in FIG. 15 is formed in such a way that asubstrate 1500, a gate electrode 1501, a gate insulating film 1502, amicrocrystalline semiconductor film 1503, a channel protection layer1504, an amorphous semiconductor film 1506, an impurity semiconductorlayer 1507, a conductive film (a source electrode and a drain electrode)1508 are sequentially formed. Note that the manufacturing methoddescribed in Embodiment Mode 1 is used. The thickness of each stackedfilm is set as follows: a glass substrate having a thickness of 100 nmis used for the substrate 1500; a molybdenum (Mo) film having athickness of 150 nm is used for the gate electrode 1501; a siliconnitride (Si₃N₄) film having a thickness of 300 nm is used for the gateinsulating film 1502; a silicon film in a microcrystalline state havinga thickness of 10 nm is used for the microcrystalline semiconductor film1503; a silicon nitride (Si₃N₄) film having a thickness of 90 nm is usedfor the channel protection layer 1504; a silicon film in an amorphousstate having a thickness of 200 nm is used for the amorphoussemiconductor film 1506; an amorphous silicon film having a thickness of50 nm to which phosphorus is added is used for the impuritysemiconductor layer 1507; and a molybdenum (Mo) film having a thicknessof 150 nm is used for the conductive film 1508. Note that the length ina channel length direction of the microcrystalline semiconductor film1503 and the channel protection layer 1504 is set at 10 μm, and thelength of the amorphous semiconductor film 1506 which is partlysuperposed over the end portions of the channel protection layer 1504 isset at 200 nm. In addition, device simulation is performed based onphysical characteristics of each stacked film.

Parameters for the device simulation of the amorphous semiconductor filmwhich forms the amorphous semiconductor film 1506 are set at thefollowing numerical values.

The numerical values are as follows: the state density of anacceptor-type defect level (tail distribution) at the conduction bandedge satisfies (nta=3.0E21 [/cm³ eV]); the state density of a donor-typedefect level (tail distribution) at the valence band edge satisfies(ntd=4.0E20 [/cm³ eV]); attenuation coefficient of the state density ofan acceptor-type defect level (tail distribution) satisfies (wta=0.025[eV]); attenuation coefficient of the state density of a donor-typedefect level (tail distribution) satisfies (wtd=0.05 [eV]); the statedensity of an acceptor-type defect level (bump distribution) at the peakposition satisfies (nga=5.0E17 [/cm³ eV]); the state density of adonor-type defect level (bump distribution) at the peak positionsatisfies (ngd=5.0E17 [/cm³ eV]); the peak position of an acceptor-typedefect level (bump distribution) satisfies (ega=0.28 [eV]); the peakposition of a donor-type defect level (bump distribution) satisfies(egd=0.79 [eV]); the attenuation coefficient of the state density of anacceptor-type defect level (bump distribution) satisfies (wga=0.1 [eV]);the attenuation coefficient of the state density of a donor-type defectlevel (bump distribution) satisfies (wgd=0.2 [eV]); the capture-crosssection for electrons in the tail of an acceptor level satisfies(sigtae=3.0E-15 [cm²]); the capture-cross section for holes in the tailof an acceptor level satisfies (sigtah=3.0E-13 [cm²]); the capture-crosssection for electrons in the tail of a donor level satisfies(sigtde=3.0E-13 [cm²]); the capture-cross section for holes in the tailof a donor level satisfies (sigtdh=3.0E-15 [cm²]); the capture-crosssection for electrons in a Gaussian distribution of acceptor satisfies(siggae=3.0E-15 [cm²]); the capture-cross section for holes in aGaussian distribution of acceptor satisfies (siggah=3.0E-13 [cm²]); thecapture-cross section for electrons in a Gaussian distribution of donorsatisfies (siggde=3.0E-13 [cm²]); and the capture-cross section forholes in a Gaussian distribution of donor satisfies (siggdh=3.0E-15[cm²]).

Parameters of a silicon film in a microcrystalline state which forms themicrocrystalline semiconductor film 1503 are set at the followingnumerical values. Note that the defect density of the silicon film in amicrocrystalline state is set at a tenth of a silicon film in anamorphous state.

The numerical values are as follows: the state density of anacceptor-type defect level (tail distribution) at the conduction bandedge satisfies (nta=2.0E21 [/cm³ eV]); the state density of a donor-typedefect level (tail distribution) at the valence band edge satisfies(ntd=4.0E19 [/cm³ eV]); the state density of an acceptor-type defectlevel (bump distribution) at the peak position satisfies (nga=9.0E17[/cm³ eV]); and the state density of a donor-type defect level (bumpdistribution) at the peak position satisfies (ngd=5.0E17 [/cm³ eV]).Other parameters are the same parameters as those of the amorphoussilicon film.

In FIG. 16, device simulation results of the transistor shown in FIG. 15are shown. A curve 1601 of FIG. 16 shows changes of current (Id) flowinga drain electrode, which corresponds to voltage (Vg) that is applied toa gate electrode when 0 V is applied to a source electrode and 14 V isapplied to a drain electrode of the transistor shown in FIG. 15. Inaddition, a curve 1602 of FIG. 16 shows changes of current (Id) flowingthe drain electrode, which corresponds to voltage (Vg) that is appliedto the gate electrode when 0 V is applied to the source electrode and 14V is applied to the drain electrode in the case where the region of thechannel protection layer 1504 in FIG. 15 is an amorphous silicon film.In addition, a curve 1603 of FIG. 16 shows changes of current (Id)flowing the drain electrode, which corresponds to voltage (Vg) appliedto the gate electrode when 0 V is applied to the source electrode and 1V is applied to the drain electrode of the transistor shown in FIG. 15.Further, a curve 1604 of FIG. 16 shows changes of current (Id) flowingthe drain electrode, which corresponds to voltage (Vg) applied to thegate electrode when 0 V is applied to the source electrode and 1 V isapplied to the drain electrode in the case where the region of thechannel protection layer 1504 in FIG. 15 is an amorphous silicon film.

From the current-voltage characteristics of the transistor shown in FIG.16, it is found that, with the transistor structure of the presentinvention, the amount of current when the transistor is turned off canbe decreased without changing the amount of current when the transistoris turned on without depending on voltage between the source electrodeand the drain electrode. It is also found that the subthreshold swing (Svalue) which is a characteristic of Id with respect to Vg is improved inFIG. 16. The improvement of characteristics of this transistor occursbecause the thickness of a channel formation region of the transistor issmall, whereby S value is improved in a similar way to a completedepletion type transistor; and generation current and recombinationcurrent are not generated and the amount of current when the transistoris turned off is decreased because an insulating film is used for theupper part of the channel formation region. As described above, thepresent invention can provide a display device including a thin filmtransistor that can improve electric characteristics and decreaseoff-current. In addition, an increase in parasitic capacitance and anincrease in production cost can be suppressed while a decrease in yieldcan be suppressed.

This application is based on Japanese Patent Application serial No.2007-205615 filed with Japan Patent Office on Aug. 7, 2007, the entirecontents of which are hereby incorporated by reference.

1. A semiconductor device comprising: a gate electrode over a substrate;a gate insulating film over the gate electrode; a first semiconductorfilm over the gate electrode with the gate insulating film interposedtherebetween; a channel protection layer over the first semiconductorfilm and in contact with the first semiconductor film; a secondsemiconductor film over the gate insulating film and in contact with afirst side surface of the first semiconductor film and with a first sidesurface of the channel protection layer; a third semiconductor film overthe gate insulating film and in contact with a second side surface ofthe first semiconductor film and with a second side surface of thechannel protection layer; a first impurity semiconductor layer over thesecond semiconductor film; a second impurity semiconductor layer overthe third semiconductor film; and a source electrode and a drainelectrode over and in contact with the first impurity semiconductorlayer and the second impurity semiconductor layer, respectively, whereina thickness of the second semiconductor film is larger than a thicknessof the first semiconductor film.
 2. The semiconductor device accordingto claim 1, wherein the first semiconductor film comprisesmicrocrystalline semiconductor.
 3. The semiconductor device according toclaim 1, wherein the second semiconductor film comprises amorphoussemiconductor.
 4. The semiconductor device according to claim 1, whereinthe channel protection layer is one of a silicon nitride film and asilicon nitride oxide film.
 5. The semiconductor device according toclaim 1, further comprising: an insulating film which is in contact withthe source electrode, the drain electrode, the first impuritysemiconductor layer, the second impurity semiconductor layer, the secondsemiconductor film, and the third semiconductor film; and a wiring overthe insulating film and connected to one of the source electrode and thedrain electrode through a contact hole formed in the insulating film. 6.The semiconductor device according to claim 1, wherein a width of thegate electrode is larger than that of the first semiconductor film. 7.An electronic device comprising the semiconductor device according toclaim
 1. 8. A semiconductor device, comprising: a gate electrode over asubstrate; a gate insulating film over the gate electrode; a firstsemiconductor film over the gate electrode with the gate insulating filminterposed therebetween; a channel protection layer over the firstsemiconductor film and in contact with the first semiconductor film; asecond semiconductor film over the gate insulating film and in contactwith a first side surface of the first semiconductor film and with afirst side surface of the channel protection layer; a thirdsemiconductor film over the gate insulating film and in contact with asecond side surface of the first semiconductor film and with a secondside surface of the channel protection layer; a first impuritysemiconductor layer over the second semiconductor film; a secondimpurity semiconductor layer over the third semiconductor film; and asource electrode and a drain electrode over and in contact with thefirst impurity semiconductor layer and the second impurity semiconductorlayer, respectively, wherein a part of the first impurity semiconductorlayer, a part of the second impurity semiconductor layer, a part of thesecond semiconductor film, and a part of the third semiconductor filmare exposed outside the source electrode and the drain electrode.
 9. Thesemiconductor device according to claim 8, wherein the firstsemiconductor film comprises microcrystalline semiconductor.
 10. Thesemiconductor device according to claim 8, wherein the secondsemiconductor film comprises amorphous semiconductor.
 11. Thesemiconductor device according to claim 8, wherein the channelprotection layer is one of a silicon nitride film and a silicon nitrideoxide film.
 12. The semiconductor device according to claim 8, furthercomprising: an insulating film which is in contact with the sourceelectrode, the drain electrode, the first impurity semiconductor layer,the second impurity semiconductor layer, the second semiconductor film,and the third semiconductor film; and a wiring over the insulating filmand connected to one of the source electrode and the drain electrodethrough a contact hole formed in the insulating film.
 13. Thesemiconductor device according to claim 8, wherein a width of the gateelectrode is larger than that of the first semiconductor film.
 14. Anelectronic device comprising the semiconductor device according to claim8.
 15. A semiconductor device, comprising: a gate electrode over asubstrate; a gate insulating film over the gate electrode; a firstsemiconductor film over the gate electrode with the gate insulating filminterposed therebetween; a channel protection layer over the firstsemiconductor film and in contact with the first semiconductor film; asecond semiconductor film over the gate insulating film and in contactwith a first side surface of the first semiconductor film and with afirst side surface of the channel protection layer; a thirdsemiconductor film over the gate insulating film and in contact with asecond side surface of the first semiconductor film and with a secondside surface of the channel protection layer; a first impuritysemiconductor layer over the second semiconductor film; a secondimpurity semiconductor layer over the third semiconductor film; and asource electrode and a drain electrode over and in contact with thefirst impurity semiconductor layer and the second impurity semiconductorlayer, respectively, wherein an end portion of the first impuritysemiconductor layer and one of end portions of the second semiconductorfilm are aligned with each other over the gate electrode.
 16. Thesemiconductor device according to claim 15, wherein the firstsemiconductor film comprises microcrystalline semiconductor.
 17. Thesemiconductor device according to claim 15, wherein the secondsemiconductor film comprises amorphous semiconductor.
 18. Thesemiconductor device according to claim 15, wherein the channelprotection layer is one of a silicon nitride film and a silicon nitrideoxide film.
 19. The semiconductor device according to claim 15, furthercomprising: an insulating film which is in contact with the sourceelectrode, the drain electrode, the first impurity semiconductor layer,the second impurity semiconductor layer, the second semiconductor film,and the third semiconductor film; and a wiring over the insulating filmand connected to one of the source electrode and the drain electrodethrough a contact hole formed in the insulating film.
 20. Thesemiconductor device according to claim 15, wherein a width of the gateelectrode is larger than that of the first semiconductor film.
 21. Anelectronic device comprising the semiconductor device according to claim15.
 22. A method for manufacturing a semiconductor device comprisingsteps of: forming a gate electrode over a substrate; forming a gateinsulating film on the gate electrode; forming a first semiconductorfilm on the gate insulating film; forming an insulating film on and incontact with the first semiconductor film; etching the firstsemiconductor film and the insulating film using a mask, thereby forminga semiconductor island and a channel protection layer; forming a secondsemiconductor film over the gate insulating film and on a side surfaceof the semiconductor island and on a side surface of the channelprotection layer; forming an impurity semiconductor layer on the secondsemiconductor film; forming a conductive layer on the impuritysemiconductor layer; and etching the conductive layer, the impuritysemiconductor layer, and the second semiconductor film, thereby formingsource and drain electrodes.
 23. The method according to claim 22,wherein the semiconductor island comprises microcrystallinesemiconductor.
 24. The method according to claim 22, wherein the secondsemiconductor film comprises amorphous semiconductor.
 25. The methodaccording to claim 22, wherein a thickness of the second semiconductorfilm is larger than a thickness of the first semiconductor film.
 26. Themethod according to claim 22, wherein one of end portions of the etchedimpurity semiconductor layer and one of end portions of the etchedsecond semiconductor film are aligned with each other over the gateelectrode.
 27. The method according to claim 22, wherein a width of thegate electrode is larger than that of the semiconductor island.
 28. Themethod according to claim 22, further comprising steps: forming a secondinsulating film in contact with the source and drain electrodes, theimpurity semiconductor layer, and the second semiconductor film; andforming a wiring over the second insulating film, the wiring connectedto one of the source and drain electrodes through a contact hole formedin the second insulating film.